1. Field of the Invention
The invention relates generally to Global Positioning System (GPS) receivers and more particularly to a method for computing a precise relative location using a single differential GPS receiver that acts as a reference station by generating differential correction terms, and then subsequently as a remote receiver unit by using the differential correction terms.
2. Background
The Global Positioning System (GPS) was established by the United States government, and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz., denoted as L1 and L2 respectively. These signals include timing patterns relative to the satellite""s onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites, an ionosphere model, and other useful information. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error.
In standalone GPS systems that determine a receiver""s position coordinates without reference to a nearby reference receiver, the process of position determination is subject to errors from a number of sources. These include errors in the GPS satellite""s clock reference, the location of the orbiting satellite, ionosphere induced propagation delay errors, and troposphere refraction errors. A discussion of these sources of error is given in more detail in U.S. Pat. No 5,828,336 by Yunck, et al.
To overcome the errors of the standalone GPS system, many kinematic positioning applications, including applications of GPS to precision farming, have made use of data from multiple GPS receivers. Typically, in such applications, a reference receiver, located at a reference site having known coordinates, receives the GPS satellite signals simultaneously with the receipt of signals by a remote receiver. Depending on the separation distance between the two GPS receivers, many of the errors mentioned above will affect the satellite signals equally for the two receivers. By taking the difference between signals received both at the reference site and at the remote location, the errors are effectively eliminated. This facilitates an accurate determination of the remote receiver""s coordinates relative to the reference receiver""s coordinates.
The technique of differencing signals from two or more GPS receivers to improve accuracy is known as differential GPS (DGPS). Differential GPS has been well described in literature with two examples cited here: 1) xe2x80x9cGlobal Positioning System: Theory and Applicationsxe2x80x9d, volume 2, pp. 3-49, 81-114, edited by Parkinson and Spilker; and 2) xe2x80x9cGPS Theory and Practicexe2x80x9d, Third edition, pp. 132-143 by Hofiann-Wellenhof, Lichtenegger and Collins. Differential GPS has taken on many forms and has been an object of a number of patents (for example, see U.S. Pat. No. 4,812,991 by Hatch, U.S. Pat. No. 5,148,179 by Allison, U.S. Pat. No. 5,155,490 by Spradley, Jr., et al., U.S. Pat. No. 5,361,212 by Class, et al., U.S. Pat. No. 5,495,257 by Loomis, U.S. Pat. No. 5,596,328 by Stangeland, and U.S. Pat. No. 5,638,077 by Martin). It includes local DGPS systems that utilize a single reference receiver delivering corrections to one or more remote receivers and it includes Wide Area Differential GPS (WADGPS) where differential correction terms are generated by combining data from multiple reference GPS receivers spread geographically over a region of intended coverage. Several methods of WADGPS appear in U.S. Pat. No. 5,323,322 by Mueller, et al., U.S. Pat. No. 5,621,646 by Enge, et al., U.S. Pat. No. 5,828,336 by Yunck, et al., and U.S. Pat. No. 5,899,957 by Loomis. In all forms of DGPS, however, the positions obtained by the end user""s remote receiver are relative to the position(s) of the reference receivers). Thus, absolute accuracy of any DGPS system depends heavily on the accuracy at which the reference receiver locations were determined when installing or implementing the DGPS system.
Relative accuracy is often all that is desired in many applications involving GPS and in these cases, the reference location need not be extremely accurate relative to any one particular coordinate system. That is, it is not a question of determining so much exact position, but position relative to some starting point with a high degree of accuracy. For example, the primary need for swathing applications that guide farm vehicles applying pesticides or fertilizer is to be able to guide the vehicle so that, relative to an initial swath, the subsequent swaths are at a series of prescribed offsets from the original swath (or from each other). There is often no accuracy requirement on the initial swath, only that subsequent swaths be accurate relative to the initial swath.
Prior to May 1, 2000, the largest portion of the positioning error associated with non-differentially corrected (standalone) GPS resulted from the purposeful dithering of the GPS satellite""s clock; a process known as Selective Availability (SA). The U.S. Department of Defense relied on SA as a means to limit GPS accuracy of non-authorized users. Typical position errors resulting from were SA 20 to 60 meters but these errors could be more than 100 meters at times. On midnight of May 1, 2000, the intentional degradation of the Global Positioning System signals was discontinued, perhaps indefinitely.
Even without SA, absolute positioning errors caused by the ionosphere can be tens of meters and orbit-induced errors can be several meters. But these errors tend to be slowly changing, sometimes taking more than 2 hours before a stationary receiver reports a position that significantly different from past reports of position. In some applications, such as the GPS guidance of agricultural aircraft used in applying chemicals, the need for relative accuracy may be well matched to a two-hour duration. In fact, it may take less than an hour to complete a typical spraying job. Thus with SA disabled, employing the method described herein, standalone GPS systems may be used where DGPS systems (with multiple GPS receivers) were once required. For example, in certain precision agriculture applications and other arenas such as surveying or Global Information Systems (GIS) DGPS may no longer be required.
Although it is true that existing standalone GPS technologies can achieve high degrees of relative accuracy for several hours, it cannot be guaranteed. Satellites that are tracked by the GPS receiver and are used in computing the receiver""s position may set below the horizon; rise above the horizon; or they may be temporarily blocked from view by an obstruction. Such transitions often cause jumps in the receiver""s computed position that maybe larger than a particular application can tolerate. To appreciate why, it is important to understand how the GPS position is computed. In the over-determined case (which is the preferred approach to solving the GPS receiver""s position) each of the GPS satellite""s range measurements contributes to the position reported by the GPS receiver because each range measurement contains information that is mathematically combined, using a Least-Squares or similar technique, to compute the GPS receiver""s position. Furthermore, each satellite""s range measurement is corrupted by errors, such as ionosphere induced delay, and each error has a different effect on the final position. The ionosphere model most often used for standalone GPS is the Klobuchar model that is broadcast in the GPS Navigation message arriving from each GPS satellite and this model is often in error by one to several tens of meters. As a satellite range measurement is, for example, removed from the Least-Squares position solution, the computed position will abruptly jump due to the sudden omission of the error source, and the remaining measurements, each having its own error contribution will dictate the new position. These ionospheric delays are errors that DGPS provides the benefit of mitigating but which standalone GPS must rely only on models to combat.
Another scenario for which there are potential jumps in the receiver""s computed location is during the updates to the satellite""s orbital parameters that are broadcast in the GPS message. These orbital parameters specify the location of the GPS satellites, and ultimately, the location of the GPS receiver depends on the assumed location of the GPS satellites. When the orbital parameters change, there is often an associated jump in the satellite""s apparent location, which translates to a jump in reported position of the GPS receiver. Again, when DGPS is used, these jumps are mitigated because the differential corrector for each satellite is matched to a particular set of orbital parameters. In practice, each DGPS correction has associated with it an identifier, known as the IODE, and this is a copy of the orbital parameter""s identifier. Simple xe2x80x9cbook-keepingxe2x80x9d, which involves the caching of orbital parameters and their identifiers, assures that the differential correction terms are matched to the orbit parameters used in computing the GPS solution.
Still another source of error that can cause drift or jumps in standalone GPS positions are troposphere model inaccuracies. These errors tend to be small for the most part, because the delay to the ranging signal caused by the troposphere can be modeled to less than one meter of error using relatively simple models.
In conclusion, many applications that previously required DGPS while SA was employed still require some form of DGPS. The need is even more pronounced for aerial applications where frequent banking of the aircraft causes satellites to continuously enter into, or exit out of view of the GPS receiver. Without some form of differential correction, the resulting computed position jumps would be intolerable.
The fact that SA is disabled has brought forth new opportunities. Fortunately, all remaining sources of error in a GPS position computation are slowly varying in nature and the variation is somewhat predictable. For example, as a satellite rises in the sky, the transmitted signal, as viewed from a GPS receiver, contains different levels of ionospheric distortion. Modeling the ionosphere as a simple shell, results in a reasonable estimate of how the signal varies. Such a model states (see, for example RTCA/DO-229A, page A-45) that the ionosphere-induced delay is roughly three times larger for satellites viewed near the horizon than it is for satellite""s viewed near the zenith. This scaling of the ionosphere delay is known as an obliquity factor. So, if the ionosphere delay is known at one point, then using the obliquity factor, the change in ionosphere can be predicted as the satellite traverses the sky. The same concept holds true for troposphere induced delays, only the obliquity factor is different.
The atomic clocks onboard the GPS satellites are also vary stable, and any drift they may possess is modeled fairly well by parameters broadcast in the GPS navigation message. So other than jumps caused by modeling parameter updates, the drifts in GPS clocks, after applying the model can be less than a meter per day. The positions of the orbiting satellites have also been modeled to within a few of meters by parameters broadcast in the GPS navigation message. Furthermore, errors in orbit position are slow changing and to a large degree, can be treated as errors in the GPS clock.
With the sources of error being small or slowly changing, especially when assisted by simple models, it follows that we could still use differential GPS but update the differential correction terms far less often than was previously necessary (when SA was on). In fact, once every hour or so may be sufficient. However, if this is the case, then the requirement of two or more receivers to produce a differentially corrected GPS solution can be reduced to one GPS receiver. That single receiver can act both as the reference station, producing the original set of differential correction terms, and then as the remote receiver using that set differential correction terms. All the while, the receiver updates the correction terms with simple models reflecting changes in the atmospheric delays. Not only does this scheme remove the cost of a receiver, but it also eliminates the communication link between receivers. A subject, which has been previously disclosed, as in for example U.S. Pat. No. 5,345,245 by Ishikawa, et al. and U.S. Pat. No. 5,589,835 by Gildea, et al. This disclosure introduces a simple but novel approach to relative GPS positioning using a single GPS receiver unit.
Of course, with relative positioning, it is still necessary to have the position of the reference location. A matter simply addressed if relative accuracy is indeed all that is required. For first time operation in a new geographic area, the reference location may be determined as the position of the GPS receiver as computed from the ensemble of the non-differentially corrected GPS range measurements at some point prior to going into differential mode. For future use in the same area a new reference may be determined, or the location may be retrieved from computer memory (or other sources) after having returned to a mark for which this location was determined. The location would have been determined in a past operation of relative DGPS positioning.